minibuffer.el

(defun minibuffer-complete ()
  "Complete the minibuffer contents as far as possible.
Return nil if there is no valid completion, else t.
If no characters can be completed, display a list of possible completions.
If you repeat this command after it displayed such a list,
scroll the window of possible completions."
  (interactive)
  ;; If the previous command was not this,
  ;; mark the completion buffer obsolete.
  (setq this-command 'completion-at-point)
  (unless (eq 'completion-at-point last-command)
    (completion--flush-all-sorted-completions)
    (setq minibuffer-scroll-window nil))

  (cond
   ;; If there's a fresh completion window with a live buffer,
   ;; and this command is repeated, scroll that window.
   ((window-live-p minibuffer-scroll-window)
    (let ((window minibuffer-scroll-window))
      (with-current-buffer (window-buffer window)
        (if (pos-visible-in-window-p (point-max) window)
            ;; If end is in view, scroll up to the beginning.
            (set-window-start window (point-min) nil)
          ;; Else scroll down one screen.
          (with-selected-window window
            (scroll-up)))
        nil)))
   ;; If we're cycling, keep on cycling.
   ((and completion-cycling completion-all-sorted-completions)
    (minibuffer-force-complete)
    t)
   (t (pcase (completion--do-completion)
        (#b000 nil)
        (_     t)))))